A common form of adjustable dockboard, as disclosed in U.S. Pat. No. 3,203,002, is mounted in the pit of a loading dock and includes a ramp which is hinged at its rear edge to the frame or supporting structure and is movable between a generally horizontal cross traffic position and an upwardly extended position. Hinged to the forward edge of the ramp is a lip which is pivoted between a downwardly hanging, pendant position and an extended position where it forms an extension to the ramp. In mechanically operated dockboards of this type, a counterbalancing spring assembly is employed to bias the ramp toward the upwardly inclined position, and the operator can then walk outwardly on the ramp, and the weight of the operator, in combination with the weight of the ramp, overcomes the counterbalancing spring force so that the ramp can be lowered downwardly to bring the extended lip into engagement with the bed of a carrier located in front of the dock.
With an upwardly counterbalanced dockboard, in which the counterbalancing force is greater than the moment of the ramp about its hinge connection, a uni-directional holddown mechanism connects the ramp to the supporting structure, and while permitting free downward movement of the ramp, prevents upward movement of the ramp through the force of the counterbalancing spring, unless manually released.
The lip is pivoted to its extended position as the ramp is moved upwardly, or alternately, as the ramp is walked down, so that the lip will be in an extended position to engage the bed of a carrier as the ramp moves downwardly. In either case, a counterbalancing spring can be associated with the lip to aid in moving the lip from the pendant position toward the extended position, as described in U.S. Pat. No. 3,203,002.
In a hydraulically actuated dockboard, a hydraulic cylinder unit is connected between the ramp and the supporting structure. By supplying hydraulic fluid to the cylinder unit, the ram will be extended to pivot the ramp upwardly to the inclined position. By cutting off the supply of hydraulic fluid to the cylinder unit, the ramp will descend by gravity to bring the extended lip into engagement with the bed of the truck or carrier.
Gas springs have been used in the past to counterbalance relatively light weight items, such as tail-gates on automobiles, desk tops and the like. With the conventional gas spring, a gas such as nitrogen, is contained within the closed cylinder and a piston is mounted for sliding movement within the cylinder, with the piston rod being connected to the object to be counterbalanced. The gas spring unit provides a biasing or counterbalancing force acting to urge the piston rod to the extended position due to the differential in exposed area on opposite faces of the piston. The piston can include one or more orifices, and by including a quantity of hydraulic fluid, such as oil, in the cylinder, a cushioning effect is obtained at the ends of the stroke of travel of the piston.